How To Measure Drive Shaft Angles
We try to define a slope as the relationship of a single line or plane in relationship to the horizon and an angle is the relationship of one slope to another. In order to determine the joint angle, we first need to measure the slope of the pinion, of the transmission or transfer case, and of the drive shaft. The differences are the net joint angles. For example, if your drive shaft is 20 degrees and your transfer case is 5 degrees down, your joint angle is 15 degrees. If your drive shaft is 20 and your transfer case is 5 degrees up, your joint angle is 25 degrees.
Get the right tool
In order to determine the slopes you will need to have some sort of tool with which to measure. There are a number of inexpensive ways that this can be done. You could purchase an "angle finder" at most hardware stores for about $10. This will usually be found in the tool department near the carpenter’s levels. It will have at least one flat base, and a circular face with graduation around it’s perimeter and a weighted plumb bob in the center.
If you don't have the right tool and you don't want to go buy one, you can download an angle tool app on any smartphone. Just search "angle" in your app store.
If you have ever seen one our business cards, you may have wondered what the little protractor on the back is for. If you attach a string at the zero point and tie a small nut or washer to the other end of the string you have just made a slope gauge!
How to measure
Measuring the slope of the drive shaft should be obvious, you just put your tool right on the drive shaft. If there is no drive shaft, you can simulate it with a piece of pipe or a string line. For our photo we have used a bungee.
For the transfer case, there are a number of ways to measure. Perhaps the best is to measure right off the output yoke or flange. If measuring the yoke, make sure you are measuring off the flat machined edge of the yoke as shown. Any flat machined surface on your engine transmission transfer case or differential will be either perpendicular or parallel to the center-line.
For the pinion/differential you can measure off the yoke or flange as stated above or you can measure off the differential cover as described below.
In our experience, the easiest way to determine the slope of the pinion is by measuring off of the differential cover bolts. All you need to do is to bridge your tool across two of the vertically spaced bolts.
Depending on how you orient your tool, you may end up with wildly different numbers. You'll have to do a little addition or subtraction to figure the number you need. If your tool says 83, subtract that from 90 and call it 7. If it says 381, subtract 360 from that and you come up with 21. Transfer cases will always be between 0 and 10. Drive shafts are usually between 10 and 40. Pinions should be between 0 and 30. If your numbers are outside of these ranges, you may be doing something wrong.
Once you figure out the slopes, you will need to add or subtract to determine the angles. Remember, the angle is the relationship of the drive shaft to whatever it is attaching to. If your transfer case or pinion is pointed toward the drive shaft, subtract the transfer case or pinion slope from the drive shaft. If the transfer case or pinion is pointed away from the drive shaft, add the two slopes together. We like making a little drawing to keep things straight. You can see some example measurements below. The black numbers are the slopes, the red numbers are the angles.
If all of this is confusing and you just want to figure out how much shim or pinion adjustment you need to make, just plug few numbers into the calculator below. The shim calculator figures out the current joint angle, takes into account how much the drive shaft slope changes as a result of the shim, and aims to achieve a 2-3 degree net joint angle. The calculator is only for double cardan rear drive shafts.
Still confused? Watch this video:
If you have questions about any of this give us a call. You'll get a real live person who can walk you through it.
Although our experience shows that measuring as described above is the most accurate way to determine angles, you can also use the chart below for a quick and rough guide to how rise and run will affect the drive shaft slope.